Subject population is comprised of 100 persons with PTSD. 50 patients will receive guanfacine and 50 patients will receive a placebo. Guanfacine is prescribed at 1 mg/day and may be increased to 2 mg/day. Samples of plasma MHPG will be collected weekly. Clinical ratings of PTSD anxiety and depression, as well as vital signs, will be assessed weekly. Repeated measures analysis of variance will assess significance of main effect of drug, time, and drug by time.